The Hash character (#) is called by many names:

• gate
• sharp
• crosshatch
• numeral sign
• number sign
• pound sign
• octothorp
• octothorpe
• "the square key" at one telephone banking company
• square (on some British Telecom services, e.g. when activating call divert)
• "iigeta jirushi" Japanese for "the sign of the well", for its resemblance to the Chinese character for water well.
• "Raute" in german
• "Gato" in Mexico and some other Spanish-speaking countries ("gato" is the name for tic-tac-toe)
• "Michi" in Peru (Mich is a common name for a cat, and cat in Spanish is "gato")
• Dièse in French

Calling # the pound sign comes from the archaic practice of using a "#" suffix to tag pound weights on bills of lading. Outside of the US, the term pound sign is used to refer to the pound currency.

See also:

• Wikipedia
• Online Computing Dictionary
• Collaborative International Dictionary of English